1. Field of the Invention
This invention relates to a low-profile unsaturated polyester resin composition which shrinks only sparingly in curing and allows production of a shaped article excelling in dimensional stability, surface property, and shock resistance.
The fiber-reinforced plastic (FRP) materials produced by incorporating such a fibrous reinforcing material as glass fibers in unsaturated polyester resin and curing the resultant composite material are excellent in mechanical strength, chemical resistance, heat resistance, and waterproofness. Owing to these outstanding characteristics, they find extensive utility in household commodities such as bathtubs, sewage tanks, and water tank panels, and automobile parts, electric parts, and other industrial materials.
2. Prior Art Statement
The trend in automobiles in recent years has been toward lower in weight and design diversification. Efforts are being made to realize adoption of FRP's in such automobile outer panels as engine hoods, roofs, and trunk lids and such exterior parts as spoilers and air-intakes. For the production of these parts, sheet molding compounds (SMC) or bulk molding compounds (BMC) which excel in workability and moldability are generally used. These shaped articles are desired to be further enhanced in dimensional stability, shock resistance, and surface property.
Unsaturated polyester resins generally shrink to a large extent in curing. The shaped articles of such resins, therefore, are liable to develop cracks, warps, undulations, and protrusion of the glass fibers used therein for reinforcement. For the purpose of overcoming these drawbacks, the practice of incorporating in the unsaturated polyester resins such thermoplastic resins as polystyrene, polymethyl methacrylate, and polyvinyl acetate as shrinkage inhibitor has been common.
As regards conventional measures for the improvement of unsaturated polyester resins in shock resistance and surface property, the most basic method comprises incorporating thermoplastic elastomers, typically a styrene-butadiene block copolymer, in the unsaturated polyester resins. Japanese Patent Public Disclosure SHO 48(1973)-34289 and Japanese Patent Public Disclosure SHO 49(1974)-30480, for example, disclose methods which incorporate a styrene-butadiene block copolymer in the unsaturated polyester resins. In these methods, however, rapid separation of the thermoplastic elastomer occurs while the various component materials are being blended or while the produced composition is being used as a molding material such as SMC. As a result, the resultant composition loses uniformity and the separated elastomer soils the metallic mold and produces scumming on the surface of a shaped article.
For the purpose of overcoming this drawback, Japanese Patent Public Disclosure SHO 52(1977)-148588 and Japanese Patent Public Disclosure SHO 54(1979)-130653 propose methods for the incorporation of a styrene-butadiene block copolymer containing a carboxyl group or a salt thereof at the molecular terminal thereof in the unsaturated polyester resin while Japanese Patent Publication SHO 62(1987)-16202 and Japanese Patent Publication SHO 62(1987)-16222 propose methods for the incorporation of a modified block copolymer obtained by binding a dicarboxylic acid group and/or a derivative group thereof to a styrene-butadiene block copolymer in the unsaturated polyester resin. These methods, however, are still incapable of completely solving the problem of layer separation between the thermoplastic elastomer component and the unsaturated polyester component.
Besides, the method of solving the problem of layer separation between the thermoplastic elastomer component and the unsaturated polyester component by the incorporation of a compatibilizer as a third component has been proposed. For example, Japanese Patent Public Disclosure SHO 55(1980)-135120 proposes a method for combined use of a comb-shaped copolymer consisting of polystyrene (backbone)-saturated polyester (branch) with a styrene-butadiene block copolymer. This method, however, provides substantially no solution to the problem of layer separation.
In the circumstances, the present inventors in Japanese Patent Public Disclosure SHO 58(1983)-189214 proposed a method for solving the problem of layer separation of a thermoplastic elastomer by the use of a block copolymer of polyvinyl acetate with poly(styrene+methacrylic acid) as a compatibilizer. This method is recognized to be effective to a certain extent in preventing the layer separation. Since it requires use of a large amount of a block copolymer deficient in toughness as a copatibilizer, it is incapable of producing a composition allowing formation of a shaped article improved in shock resistance and surface property.
There is a need for a low-profile unsaturated polyester resin composition which shrinks only sparingly in curing and allows production of a shaped article excelling in dimensional stability, surface property, and shock resistance.